Keep posters and casts between runs

Posters (the downloaded bytes, per size, 30 days, at most 200 MB) and
casts (people.json, 7 days, at most 2000) are stored in the user's cache
directory (os.UserCacheDir()/arr-tui) and used on the next start before
asking the servers. Expired posters are pruned at startup, casts are
written shortly after new ones arrive and on exit. Tests turn the cache
off.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ui/app.go

@@ -36,6 +36,7 @@ type App struct {
 	theme  theme         // the colors in use
 	trakt  *trakt.Client // trending, most watched and anticipated lists, casts
 	people *peopleCache  // casts loaded from Trakt
+	disk   *diskCache    // posters and casts kept between runs
 	// Called (on the UI goroutine) a moment after a release was grabbed.
 	afterGrab []func()
 	// The Movies and Series views, to refresh their Trakt lists.
@@ -180,7 +181,9 @@ func New(cfg *config.Config) *App {
 		time.Duration(cfg.UI.RefreshSeconds)*time.Second)
 	a.downloads = downloads
 	a.trakt = trakt.NewClient(cfg.Trakt.ClientID, "arr-tui")
+	a.disk = newDiskCache()
 	a.people = newPeopleCache()
+	a.loadSavedPeople()
 	if u := os.Getenv("ARR_TUI_TRAKT_URL"); u != "" {
 		a.trakt.SetBaseURL(u) // another Trakt API, for development (dev/screenshots)
 	}
@@ -443,7 +446,11 @@ func (a *App) Run() error {
 		if a.gfx != nil {
 			a.gfx.deleteAll()
 		}
+		if a.people.dirty { // casts fetched in the last seconds
+			_ = a.disk.savePeople(a.peopleSnapshot())
+		}
 	}()
+	go a.disk.prunePosters()
 	// The screen draws through the theme (see themedScreen).
 	s, err := tcell.NewScreen()
 	if err != nil {